You could make it so when in AI is in a position where it would be close to considering selling a province normally, a player would be able to offer some extra amount as an incentive to push them over the edge. Basically add money as a modifier to the factors that affect acceptance of the action. AIs that would never under any circumstances sell a province would have -1000 as usual. So you'd go to the diplomatic macro page, see an AI with -5 to sell a province (normally valued at, say, 300 ducats), and be able to offer 300 x 5 = 1500 instead.
Effectively you'd have almost no change from the number of provinces being sold now. layers would just be able to proactively see an AI in a position to sell a province and offer extra to get it themselves, rather than just sitting around hoping the AI decided to offer it to the player.